America's war on drugs took a major step toward ceasefire on Thursday, as a bipartisan group of senators voted to move forward with the first substantial cut in mandatory minimum sentences, and as the Justice Department made it known that President Obama is looking to commute the sentences of more existing prisoners.

Legislation proposing dramatic reductions in federal prison terms for non-violent drug offenders, and an end to the hundred-fold disparity between crack and powder cocaine sentencing advanced through the Senate judiciary committee by a vote of 13 to five.

The support of a number of right-wing Republicans, including Rand Paul, Mike Lee and Ted Cruz – who joined as a last-minute co-sponsor – gives significant momentum to a full Senate vote on the bill, which is mirrored by a House proposal from Republican Raul Labrador and Democrat Bobby Scott.

Action in Congress follows a call by Attorney General Eric Holder in August for a new approach to the war on drugs, which began in the early 1970s and has left the US with 25% of the world's prisoners.

Senate Democratic whip Dick Durbin, a co-sponsor of the bill, known as the Smarter Sentencing Act, said it would help tackle a 500% increase in the federal prison population since the early 1980s, half of whom are drug offenders. “It has been 30 years since we reviewed some of these laws,” he said. “There has also been a 1,100% increase in spending on incarceration.”

The bill, which was introduced by Durbin and judiciary committee chair Patrick Leahy, cuts some mandatory minimum sentences by more than half, gives judges greater discretion to exempt individual cases, and will allow parole hearings to redress a gulf between crack and powder cocaine punishments that has often meant harsher sentences for African-American drug users than white ones.

The vote came as the Justice Department took the unusual step of asking defence lawyers to recommend clients serving long drug sentences for future presidential pardons.

Though only the first step towards making the legislation law, the bill’s passage through the judiciary committee was greeted enthusiastically by campaigners for sentencing reform.

“The tide has turned against punitive drug policies that destroy lives and tear families apart,“ said Bill Piper, director of national affairs at the Drug Policy Alliance. “From liberal stalwarts to Tea Party favourites there’s now consensus that our country incarcerates too many people, for too much time, at too much expense to taxpayers.”

But the obstacles ahead also became clear as several Democrats expressed concern about aspects of the bill, including New York Senator Chuck Schumer, who said he was worried that reducing mandatory sentences for lower level offenders could make it harder for prosecutors to force them to testify against gang leaders.

“I am voting to move this bill out [of committee] but I have some reservations,” he said. “The change to lower end sentencing could be dangerous to crime fighting.”

Republican Chuck Grassley also claimed the bill could increase, rather than decrease, racial disparity in prison sentencing by giving judges more discretion. “A lack of mandatory minimums [in the past] led to some judges who were far too lenient,” he told the committee.

Drug sentencing reforms are backed by most law enforcement groups, but the National Association of Assistant US Attorneys broke ranks shortly before the vote with a letter opposing the bill.

“We do not join with those who regard our federal system of justice as ‘broken’ or in need of major reconstruction,” it said. “Instead, we consider the current federal mandatory minimum sentence framework as well-constructed and well worth preserving.”

Testifying before the committee on Wednesday, Holder reiterated his call for Congress to take the lead and said he was looking forward to working with Senators Leahy and Paul to push legislation forward.

“As part of the 'smart on crime' approach, I mandated a significant change to the Justice Department’s charging policies to ensure that people accused of certain low-level federal drug crimes will face sentences appropriate to their individual conduct – and that stringent mandatory minimum sentences will be reserved for the most serious criminals,” he said.

“Alongside other important reforms, this change will make our criminal justice system not only fairer, but also more efficient. And it will complement proposals like the bipartisan Smarter Sentencing Act, introduced by Senators Dick Durbin and Mike Lee, which would give judges more discretion in determining appropriate sentences for people convicted of certain federal drug crimes.”